See as you are getting NSIT Delhi CSE , it is not bad. You should go for it. But here you  are also telling that If you drop this year then as you say you will get IIT Delhi CSE.

I don't know it is confidence or over-confidence. But if you are confident about yourselves then you can drop no problem. But to be on safe side i would suggest that you take admission in NSIT and then prepare for next year as first year syllabus is more or less similar to 12th syllabus.

But it is your life so you have to decide. I am just suggesting , You take your own decision.

Thank You.